In lithography for device manufacture, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. Patterns of smaller features having critical dimensions (CD) at nano-scale allow for greater concentrations of device or circuit structures, yielding potential improvements in size reduction and manufacturing costs for electronic and other devices. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography.
So-called imprint lithography generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
For both photolithography and for imprint lithography, it is desirable to provide high resolution patterning of surfaces, for example of an imprint template or of other substrates, and chemical resists may be used to achieve this.
The use of self-assembly of a block copolymer (BCP) has been considered as a potential method for improving the resolution to a better value than obtainable by prior art lithography methods or as an alternative to electron beam lithography for preparation of imprint templates.
A self-assemblable block copolymer is a compound useful in nanofabrication because it may undergo an order-disorder transition on cooling below a certain temperature (order-disorder transition temperature TOD) resulting in phase separation of copolymer blocks of different chemical nature to form ordered, chemically distinct domains with dimensions of tens of nanometres or even less than 10 nm. The size and shape of the domains may be controlled by manipulating the molecular weight and composition of the different block types of the copolymer. The interfaces between the domains may have a line width roughness of the order of 1-5 nm and may be manipulated by modification of the chemical compositions of the blocks of the copolymers.
The feasibility of using thin films of block copolymers as self-assembling templates was demonstrated by Chaikin and Register, et al., Science 276, 1401 (1997). Dense arrays of dots and holes with dimensions of 20 nm were transferred from a thin film of poly(styrene-block-isoprene) to a silicon nitride substrate.
A block copolymer comprises different blocks, each comprising one or more identical monomers, and arranged side-by side along the polymer chain. Each block may contain many monomers of its respective type. So, for instance, an A-B block copolymer may have a plurality of type A monomers in the (or each) A block and a plurality of type B monomers in the (or each) B block. An example of a suitable block copolymer is, for instance, a polymer having covalently linked blocks of polystyrene (PS) monomer (hydrophobic block) and polymethylmethacrylate (PMMA) monomer (hydrophilic block). Other block copolymers with blocks of differing hydrophobicity/hydrophilicity may be useful. For instance a tri-block copolymer such as (A-B-C) block copolymer may be useful, as may an alternating or periodic block copolymer e.g. [-A-B-A-B-A-B-]n or [-A-B-C-A-B-C]m where n and m are integers. The blocks may be connected to each other by covalent links in a linear or branched fashion (e.g., a star or branched configuration).
A block copolymer may form many different phases upon self-assembly, dependent upon the volume fractions of the blocks, degree of polymerization within each block type (i.e. number of monomers of each respective type within each respective block), the optional use of a solvent and surface interactions. When applied in a thin film, the geometric confinement may pose additional boundary conditions that may limit the phases formed. In general spherical (e.g. cubic), cylindrical (e.g. tetragonal or hexagonal) and lamellar phases (i.e. self-assembled phases with cubic, hexagonal or lamellar space-filling symmetry) are practically observed in thin films of self-assembled block copolymers, and the phase type observed may depend upon the relative molecular volume fractions of the different polymer blocks. For instance, a molecular volume ratio of 80:20 will provide a cubic phase of discontinuous spherical domains of the low volume block arranged in a continuous domain of the higher volume block. As the volume ratio reduces to 70:30, a cylindrical phase will be formed with the discontinuous domains being cylinders of the lower volume block. At 50:50 ratio, a lamellar phase is formed. With a ratio of 30:70, an inverted cylindrical phase may be formed and at a ratio of 20:80, an inverted cubic phase may be formed.
Suitable block copolymers for use as a self-assemblable polymer include, but are not limited to, poly(styrene-b-methylmethacrylate), poly(styrene-b-2-vinylpyridone), poly(styrene-b-butadiene), poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethyleneoxide), poly(ethyleneoxide-b-isoprene). The symbol “b” signifies “block” Although these are di-block copolymer examples, it will be apparent that self-assembly may also employ a tri-block, tetrablock or other multi-block copolymer.
The self-assembled polymer phases may orient with symmetry axes parallel or perpendicular to the substrate and lamellar and cylindrical phases are interesting for lithography applications, as they may provide a resist to form line and space patterns and hole arrays, respectively, when oriented with their domains lying side-by-side on a substrate, and may provide good contrast when one of the domain types is subsequently etched.
Two methods used to guide or direct self-assembly of a polymer, such as a block copolymer, onto a surface are graphoepitaxy and chemical pre-patterning, also called chemical epitaxy. In the graphoepitaxy method, self-organization of block copolymer is guided by topological pre-patterning on the substrate. Lamellar self-assembled block copolymer can form parallel linear patterns with adjacent lines of the different polymer block domains in the enclosures or trenches defined by one or more side walls of the graphoepitaxy template. For instance if the block copolymer is a di-block copolymer with A and B blocks within the polymer chain, where A is hydrophilic and B is hydrophobic in nature, the A blocks may assemble into domains formed adjacent to a side-wall of a trench if the side-wall is also hydrophilic in nature. Resolution may be improved over the resolution of the graphoepitaxy template by a side wall being spaced to fit several domains of the block copolymer side-by-side. For hexagonal or tetragonal (cylindrical) ordered patterns, the graphoepitaxy features may be pillars standing in place of cylindrical domains of the ordered pattern of the block copolymer.
In the chemical pre-patterning method (referred to herein as chemical epitaxy), the self-assembly of block copolymer domains is guided by a chemical pattern (i.e. a chemical epitaxy template) on the substrate. Chemical affinity between the chemical pattern and at least one of the types of copolymer blocks within the polymer chain may result in the precise placement (also referred to herein as “pinning”) of one of the domain types onto a corresponding region of the chemical pattern on the substrate. For instance if the block copolymer is a di-block copolymer with A and B blocks, where A is hydrophilic and B is hydrophobic in nature, and the chemical epitaxy pattern may comprise a hydrophobic region on a hydrophilic surface, the B domain may preferentially assemble onto the hydrophobic region. As with the graphoepitaxy method of alignment, the resolution may be improved over the resolution of the patterned substrate by the block copolymer pattern subdividing the spacing of the pre-patterned features on the substrate (so-called density or pitch multiplication). As was the case with graphoepitaxy, chemical pre-patterning is not limited to a linear pre-pattern; for instance the chemical epitaxy template may be in the form of a 2-D array of dots suitable as a pattern for use with a cylindrical (e.g. hexagonal or square pattern) phase-forming block copolymer. Graphoepitaxy and chemical pre-patterning may be used, for instance, to guide the self-organization of lamellar or cylindrical phases, so that the different domain types are arranged side-by-side on a surface of a substrate.
Typically, the height of features of a graphoepitaxy template may be of the order of the thickness of the block copolymer layer to be ordered, so may be, for instance, from 20 nm to 150 nm whereas for a chemical epitaxy template, the height difference between adjacent regions of a chemical epitaxy template will typically be less than about 15 nm, say less than about 10 nm or even less than about 5 nm in order to reduce or minimize likelihood of defect formation.